The present invention relates generally to techniques which are effectively applied to an on-chip voltage controlled oscillator circuit (VCO) that can switch oscillating frequencies, and more particularly, to techniques which are effectively utilized in high frequency semiconductor integrated circuits and radio communication systems for demodulating a reception signal and modulating a transmission signal in radio communication apparatuses, for example, a portable telephone and the like which can transmit and receive signals in a plurality of bands.
A radio communication system such as a portable telephone uses a PLL (phase locked loop) circuit which has a VCO for generating an oscillating signal at a predetermined frequency. The oscillating signal is combined with a reception signal and a transmission signal. Conventional portable telephones include a dual-band portable telephone which can handle signals in two frequency bands, for example, a GSM (Global System for Mobile Communication) signal in a band of 880-915 MHz and a DCS (Digital Cellular System) signal in a band of 1710-1785 MHz. Some dual-band portable telephones are designed to support two different bands with a single PLL circuit by switching the frequency of the PLL circuit.
In recent years, however, a need exists for a triple-band portable telephone which can handle, for example, a PCS (Personal Communication System) signal in a band of 1850-1915 MHz in addition to the GSM and DCS signals. It is also contemplated that the portable telephones are required to support a larger number of bands in the future.
For a high frequency semiconductor integrated circuit (hereinafter called the “high frequency IC”) designed to modulate a transmission signal and demodulate a reception signal, for use in such a portable telephone which can support a plurality of bands, a direct conversion system is effective from a viewpoint of a reduction in the number of parts. While the direct conversion system is relatively easy in supporting a plurality of bands, a VCO should be capable of oscillating over a wide frequency range. In this event, when a single VCO is used with the intention to cover the overall frequency range, the resulting VCO would be extremely sensitive to a control voltage applied thereto, and therefore vulnerable to extraneous noise and fluctuations in a power supply voltage.
On the other hand, a reduction in the number of parts may be effectively accomplished by forming a VCO, which has been typically fabricated in a module separate from a high frequency IC in many cases, on the same semiconductor chip on which the high frequency IC is fabricated. However, since an on-chip VCO manufactured by the current technologies experiences large variations in the absolute value of the oscillating frequency, the on-chip VCO must be provided with a function of correcting the oscillating frequency after the manufacturing. However, if the variations are corrected by trimming based on a mask option or a bonding wire option, typically used in conventional semiconductor integrated circuits, the cost is inevitably increased.
To solve these problems, some of inventors have previously developed a communication semiconductor integrated circuit (high frequency IC) comprising a PLL circuit, and filed a PCT application No. PCT/GB2002/005152 on Nov. 13, 2002 based on U.K. Patent Application No. 0127537.9. The PCT application is pending but not admitted as the prior art. In this communication semiconductor integrated circuit, an oscillator circuit (RFVCO) for generating a high frequency signal for use in transmission and reception is desired to operate in a plurality of bands. The oscillating frequency of the oscillator circuit is measured in each of the bands while the oscillator circuit is applied with a control voltage fixed at a predetermined value, and stored in a storage circuit. A set value for specifying the frequency, applied during a PLL operation, is compared with the measured frequency values stored in the storage circuit to determine a band which is actually used in the oscillator circuit from the result of the comparison. The resulting communication semiconductor integrated circuit does not have a higher sensitivity to the control voltage, and therefore is less susceptible to extraneous noise and fluctuations in the power supply voltage even with a wider frequency range available for the VCO to oscillate for supporting a plurality of communication schemes. In addition, the communication semiconductor integrated circuit can automatically correct variations in the oscillating frequency of the VCO in an internal circuit.